The cell wall of Saccharomyces cerevisiae (S. cerevisiae) is composed of a series of widely studied polysaccharides. Glucan, a major component, is a glucose homopolymer consisting of repeating (1-3)beta-D residues. In addition to structural glucans, the cell wall contains roughly equal amounts of "mannan", a mannose-containing polymer. Chitin, a (1-4)-beta-D-linked polymer of N-acetylglucosamine, comprises only about 1% of the total cell wall. This minor component has drawn interest due to its localized deposition in the septa of budding cells.
The rigid architecture of the wall, which dictates the characteristic shape of the cell, must accommodate changes in morphology that accompany processes such as budding, sporulation, or "shmoo" formation. Structural alterations essential to these processes may be accomplished in part through the regulated catabolism of the cell wall. Consistent with this hypothesis, a number of autolytic hydrolases have been reported to be associated with the yeast cell envelope, including several exo- and endoglucanases and an endochitinase activity. Additionally, .alpha.-mannosidase activities capable of hydrolyzing mannose oligosaccharides or p-nitrophenyl-.alpha.-D-mannopyranoside have been found in S. cerevisiae.
A large number of human and other proteins have been produced in host cells by transfecting these cells with DNA encoding these proteins and growing the recombinant cells under conditions favorable for the expression of the protein. In some cases, the proteins are secreted by the cells into the cell culture medium, and must be separated from the culture medium and the other components, such as cell waste products, cell debris and proteins or other material, which also has collected in the medium. To be useful, the biological activity of the protein must be preserved. Thus, recovery conditions must be mild enough to preserve the biological activity of the protein, but, at the same time, suitable for effective separation of the protein from contaminants in the medium. Purity is often an important consideration, especially for pharmaceutical application.
Recovery of proteins in biologically active form from cell culture medium presents a number of problems. For example, the desired protein must be separated from other closely related proteins in the cell culture medium, such as homologous, biologically inactive proteins, which may be associated with the protein. Further, the desired protein must generally be recovered in such a way that it retains its native activity. Preferably, the process used should result in recovery of the biologically active form of the protein with a high level of purity.